Britons in the path of Hurricane Irma are being warned the “situation could deteriorate significantly” as it bears down on the US mainland.

The foreign and commonwealth office has said its ability to provide assistance may be “extremely limited” and advised those affected to make their own contingency plans.

The historic storm regained its category five status overnight before dropping back to category four on Saturday after leaving more than 20 people dead across the Caribbean.

More than six million people in Florida and Georgia have been warned to leave their homes as the hurricane continues to batter the north coast of Cuba.

Prime minister Theresa May said work was taking place with US authorities to ensure British expats and tourists in Florida are protected as millions of locals and visitors flee to safety.

But the latest travel advice issued by the government states:

Across the United States, it is important that you follow the advice of the local authorities, including any evacuation orders.

The situation could deteriorate significantly. Our ability to provide assistance may be extremely limited.

You should ensure you have your own contingency plans in place and consider your travel plans very carefully.

Aid and expertise is being provided to Britain’s territories in the region in a 32 million government cash injection.

Engineers, marines and medics are being carried on board RFA Mounts Bay, which delivered six tonnes of supplies to Anguilla and carried out repair work before moving on to the British Virgin Islands.

Those on Caribbean islands braced for a second battering, this time from Hurricane Jose, were told the storm has weakened slightly.

But forecasters warned it was still a “dangerous” category four hurricane, which is expected to come close to the devastated northern Leeward Islands on Saturday.

A hurricane warning for the Commonwealth island of Barbuda and the British territory of Anguilla has been downgraded to a tropical storm warning, while Antigua and the British Virgin Islands are on tropical storm watch.

Life-threatening wind, rain and a storm surge are expected in the Turks and Caicos Islands, another British territory, into Saturday, after it was “pummelled” by Irma on Thursday night.

The foreign office has set up a hotline for people affected by the disaster and for people whose loved ones may be affected, on 020 7008 0000.

Richard Branson has published an update on the situation in the British Virgin Islands before the new storm hits.

He said:

Governments – the UK in the British Virgin Islands, the US in the US Virgin Islands – need to do all they can to help people here who have lost their homes and in many cases will have lost their livelihoods after the storm.

Man-made climate change is contributing to increasingly strong hurricanes causing unprecedented damage. The whole world should be scrambling to get on top of the climate change issue before it is too late – for this generation, let alone the generations to come.

We are hopeful Hurricane Jose will not hit the BVI as hard as Hurricane Irma did, but urging everyone to get prepared and helping with supplies where we can.

My colleague Ed Pilkington is in Miami and has this update on the atmosphere as Hurricane Irma heads towards Florida:

Amid the relentless news of devastation left in the wake of Hurricane Irma, millions of Floridians bracing for one of the strongest storms to hit the US mainland this century will clutch at any good news, and they got some this morning.

In the 8am advisory from the National Hurricane Center, which is being watched here in South Florida with almost biblical zeal, the weather experts told us that the hurricane’s winds had slowed from about 155mph to 130mph as a little of its terrifying energy was soaked up by the landmass on the coast of Cuba.

The hope that the weather analysts gave with one hand, they took away with the other, however. “Irma is forecast to restrengthen once it moves away from Cuba,” the advisory goes on to say, “and Irma is expected to remain a powerful hurricane as it approaches Florida.”

That spells certain catastrophe now for the Florida Keys, which is expected to be struck by Irma very early on Sunday morning. Thousands of people have already left the Keys, though as the Miami Herald is reporting some die-hards are clinging on which seems rash bearing in mind the threat of a storm surge of up to 10ft.

It also spells trouble for the west coast of Florida where Irma is continuing to target given a slight westward veer in the past 24 hours. The impact could reach as far north as Tampa Bay, which hasn’t experienced a major hurricane since 1921.

Irma is now 215 miles south of Miami and moving north at a speed of 12mph. In Miami the first power outages – of 26,000 homes – have been reported, but hopes are rising that some of the worst of the damage may be avoided as the strife swings towards the west coast.

Nonetheless, the streets of the city were virtually empty on Saturday morning, and all petrol stations, supermarkets and other commercial outlets were closed. The city of 6 million people is now in a stunned state of foreboding, waiting to find out what nature has in store for the fourth-largest urban area.

The Guardian has spoken to Joe Farrar from Manchester, who is on holiday in Varadero, Cuba with his girlfriend, Helen. He says he was meant to leave Cuba on Friday, but Thomas Cook delayed his flight. He says Canadian tourists had been evacuated from the resort complex, but British tourists have been left at the hotel.

Hurricane Irma begins lashing Florida

The window for Florida residents to safely evacuate narrowed on Saturday as strong winds and outer rain bands hit the southern part of the state on a predicted path for landfall southwest of the heavily populated Miami metro area.

Hurricane Irma is currently moving along the coast of Cuba about 215 miles away from Miami, as it makes its way towards the US peninsula.

Miami, early on Saturday morning. Photograph: Joe Raedle/Getty Images

Forecasters expect the storm to pick strength back up as it moves away from Cuba adter it weakened to a category 4 level storm.

The National Weather Service said damaging winds were moving into areas including Key Biscayne and Coral Gables on Saturday morning, while gusts of up to 56 mph (90 kph) were reported on Virginia Key off Miami.

Forecasters adjusted the storm’s potential track more toward the west coast of Florida, away from the Miami metropolitan area of 6 million people, meaning “a less costly, a less deadly storm,” University of Miami researcher Brian McNoldy said.

Nevertheless, forecasters warned that its hurricane-force winds were so wide they could reach from coast to coast, testing the nation’s third-largest state, which has undergone rapid development and more stringent hurricane-proof building codes in the last decade or so.

“This is a storm that will kill you if you don’t get out of the way,” National Hurricane Center meteorologist Dennis Feltgen said. “Everybody’s going to feel this one.”

In one of the country’s largest evacuations, about 5.6 million people in Florida more than one-quarter of the state’s population were ordered to leave, and another 540,000 were ordered out on the Georgia coast. Authorities opened hundreds of shelters for people who did not leave. Hotels as far away as Atlanta filled up with evacuees.

Parts of Florida are already experiencing power outages, according to reports on social media.

Some shelters are already at full capacity and others are filling fast. Perry Stein, a reporter for the Washington Post is in west Miami dade, which is beginning to feel the affects of the hurricane.

The UK government has pledged to double any public donations made to the British Red Cross appeal for victims of Hurricane Irma.

For every £1 given towards the relief effort in the Caribbean, another £1 will be added through the department for international development’s aid match scheme, up to £3m.

Donations will support people through the region, including in the badly hit British Overseas Territories of Anguilla, British Virgin Islands and Turks and Caicos.

Priti Patel, secretary of state for international development, says:

Today we are announcing we will double any public donations which are made to the British Red Cross appeal through DFID’s Aid Match scheme.

The overwhelming generosity of the UK public in times of crisis is one of the things that makes Britain truly great.

This will help get water, food, shelter and power to those left devastated by Hurricane Irma.

This donation is on top of the £32m already pledged by the government.‎

Mike Adamson, chief executive of British Red Cross, says:

We would like to thank DFID for supporting our Hurricane Irma appeal, raising urgent funds for those affected by this devastating disaster. The most powerful hurricane ever recorded in the Atlantic has left major destruction in its wake.

Red Cross teams are on the ground now supporting people in shelters, providing first aid and doing whatever they can to help.

The pledge came after Patel visited her department’s disaster response centre in Kemble, Gloucestershire on Friday.

About 10,000 UK aid buckets and 5,000 UK solar lanterns were being prepared at the centre while she was there and it is hoped they would reach those in need in the next few days.

Patel also pledged on Friday to double the number of humanitarian experts in the region from four to eight, who are now working in the field across several islands to assess need. Further advisors are expected to arrive in the coming days.

South Florida is experiencing "damaging winds" as Irma approaches

Meteorologists have reported that damaging winds are blowing into South Florida as Hurricane Irma continues its menacing approach on the US mainland.

The National Weather Service say that damaging winds are moving into areas including Key Biscayne, Coral Gables and South Miami.

Gusts of up to 56 mph (90 kph) were reported on Virginia Key off Miami as the storm’s outer bands arrived. The centre of the storm was about 245 miles south-east of Miami early on Saturday as it raked the northern coast of Cuba.

The chairs of the UK’s foreign affairs and development select committees have asked the government to explain its response to Hurricane Irma, which has been widely deemed as inadequate.

“Experts and many in the area have been critical of the overall level of relief currently on offer as well as the apparent lack of forward-thinking once the storm’s route to Florida became more than just a possibility,” Tom Tugendhat and Stephen Twigg wrote.

On Friday following a meeting of the government’s emergency Cobra committee, May said:

I heard directly from our consul general in Miami about the support that is being given to British nationals living in Florida and also British tourists in Florida.

We are, of course, working with the US authorities to ensure that every support is available and everything can be done before Hurricane Irma reaches Florida.

The Foreign and Commonwealth Office have released an updated advisory on US travel saying that its ability to assist British people in Florida may be limited and anyone there should follow advice from local authorities.

It is unknown how many Brits are currently in Florida, a popular holiday destination for families and older people.

Across the United States it is important that you follow the advice of the local authorities, including any evacuation orders. The situation could deteriorate significantly. Our ability to provide assistance may be extremely limited. You should ensure you have your own contingency plans in place and consider your travel plans very carefully.